Tumor oxygenation is an important variable in treatment and tumors exhibit a wide range of oxygen tensions. We are investigating methods of assessing tumor oxygenation using non-invasive MR techniques. BOLD, or blood oxygen level dependent, MR imaging is sensitive to the deoxyhemoglobin content of blood. We will undertake MR imaging of tumors during hyperoxygenation with carbogen (95% O2/5% CO2). In parallel experiments, we will measure the tumor pO2 using EPR oximetry. We will determine if the BOLD response can be used to monitor tumor responses to hyperoxygenation over ranges of pO2 which relate to changes in radiation sensitivity (0-10mmHg).